U.S. patent application number 11/247710 was filed with the patent office on 2006-03-16 for structured fluid compositions for electrophoretically frustrated total internal reflection displays.
Invention is credited to Robert N. Jones, Ralph E. Kornbrekke, Saurabh S. Lawate.
Application Number | 20060056009 11/247710 |
Document ID | / |
Family ID | 34795200 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060056009 |
Kind Code |
A1 |
Kornbrekke; Ralph E. ; et
al. |
March 16, 2006 |
Structured fluid compositions for electrophoretically frustrated
total internal reflection displays
Abstract
A structured fluid composition comprising: (a) a low refractive
index liquid; (b) particles including light absorbing charged
particles such as pigments, non-light absorbing uncharged particles
such as teflon, silica, alumina and combinations thereof; and (c)
at least one additive selected from the group consisting of a
dispersant, a charging agent, a surfactant, a flocculating agent, a
polymer, and combination thereof; for use in a TIR electronic
display. The inventive composition improves the long-term
stability, response time and visible appearance of image displays
which electrophoretically frustrate total internal reflection
(TIR).
Inventors: |
Kornbrekke; Ralph E.;
(Chagrin Falls, OH) ; Lawate; Saurabh S.;
(Concord, OH) ; Jones; Robert N.; (Mentor,
OH) |
Correspondence
Address: |
Teresan W. Gilbert;The Lubrizol Corporation
Patent Dept./Mail Drop 022B
29400 Lakeland Blvd.
Wickliffe
OH
44092-2298
US
|
Family ID: |
34795200 |
Appl. No.: |
11/247710 |
Filed: |
October 11, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10764070 |
Jan 23, 2004 |
|
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11247710 |
Oct 11, 2005 |
|
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Current U.S.
Class: |
359/296 |
Current CPC
Class: |
B82Y 20/00 20130101;
B01J 13/0034 20130101; G02F 1/167 20130101; G02F 1/1677 20190101;
G02F 2203/026 20130101; B01F 17/0085 20130101; G02F 2202/36
20130101 |
Class at
Publication: |
359/296 |
International
Class: |
G02B 26/00 20060101
G02B026/00 |
Claims
1. (canceled)
2. An electronically addressable display, comprising: (a) a
transparent upper front sheet; (b) a lower sheet that is
essentially parallel to and spaced from the upper front sheet; (c)
a structured electrophoretic suspension substantially filling the
space between the sheets which structure is controlled by the
composition of the suspension, wherein the composition comprises a
low refractive index liquid; a light absorbing particles; particles
which are not light absorbing; and at least one additive selected
from the group consisting of a dispersant, a charging agent, a
surfactant, a flocculating agent, a polymer, and combination
thereof; and (d) a means for applying a voltage across the
suspension for controllably compressing the colloidal suspension
away from the inward surface of the front sheet to either form a
thin particle free liquid layer to allow total internal reflection
or a layer with a higher concentration of particles to frustrate
total internal reflection at the inward surface of light rays
passing through the front sheet.
3. The display of claim 2 wherein the composition is a structured
fluid composition comprising: (a) a low refractive index liquid;
(b) particles selected from the group consisting of light absorbing
particles; non-light absorbing particles or very low
light-absorbing, particles wherein the non-light absorbing
particles help to create a structured colloidal suspension; and
wherein the particles interact through colloidal forces without
encapsulating the fluid in isolated compartments; and (c) at least
one additive selected from the group consisting of (i) a
dispersant. (ii) a charging agent. (iii) a surfactant, (iv) a
flocculating agent, (v) a polymer, and (vi) combination thereof;
wherein the particle has a sufficient number of functional groups
of either acid or base, to allow the dispersant to form a tightly
packed mono-layer, and wherein the dispersant has the complementary
acid or basic function group to interact with the particle surface
and a molecular structure resulting in a strong interaction between
the particle surface and the dispersant to inhikbit agglomeration,
resulting in a stable suspension that is not agglomerated, having
ionically charged light absorbing particles, and forming an
interactive structure which inhibits motion therein. wherein the
liquid electrophoretic medium is comprised of substantially
fluorinated oils.
4. The display of claim 3 wherein the liquid electrophoretic medium
is comprised of substantially fluorinated oils.
5. The display of claim 3 wherein the particles of the composition
occupy from about 1 to about 75% by weight of the electrophoretic
suspension.
6. The display of claim 3 wherein the particles of the composition
comprise a blue pigment, red pigment, brown pigment, black pigment
and combinations thereof.
7-9. (canceled)
10. The display of claim 6 comprising a mixture of two or more
pigment particles to enhance the optical properties, wherein the
frustration of total internal reflection is improved by the
collective absorption of different wavelengths of light.
11. The display of claim 3 wherein the composition results in a
colloidal structure with a non-Newtonian rheology.
12. The display of claim 3 wherein the composition results in a
colloidal structure which has a yield stress.
13. The display of claim 3 wherein the particles of the composition
have a surface treatment selected from the group consisting of
reaction with an oxidizing or reducing chemical, reaction with a
chemical that covalently bonds to the surface, grafting onto the
surface with a plasma containing small molecules with various
functional groups or mixtures thereof resulting in improved
response time and as herein the dispersant forms a tightly packed
monolayer adsorbed on the particle surface resulting in less
particle agglomeration.
14. The display of claim 3 (a) wherein the particles of the
composition have a sufficient number of functional groups of either
acid or base, to allow a dispersant to form a tightly packed
mono-layer, and (b) wherein the dispersant has the complementary
acid or basic functional group to interact with the particle
surface and a molecular structure resulting in a strong interaction
between the particle surface and the dispersant to inhibit
agglomeration.
15. The display of claim 3 wherein the particles of the composition
are suspended and have at least two distinct particle size
distributions one in the range of about 200 nm to about 500 nm and
the other in the range of about 10 nm to about 100 nm.
16. (canceled)
17. The display of claim 3 wherein the dispersant has only an
acidic functional group or a basic functional group.
18. The display of claim 3 wherein the ratio of dispersant to
pigment ranges from about 0.1 to about 3.
19. The display in claim 3 wherein the concentration of pigment
particles is adjusted to maintain small particle separation
distance in a homogeneous dispersion so the distance that particles
must move to produce a color change in TIR is small, in a fast
response time in producing an image.
20. The display of claim 3 where the pigment concentration is high
enough to enable rapid color change in an electric field.
21. (canceled)
22. (canceled)
23. The display in claim 3 wherein the charging agent, dispersant
or surfactant forms inverse micelles which increase the particle
charge thereby improving the structure and response time of the
mixture.
24. The display of claim 3 wherein the light absorbing particles
are pigment and the non light absorbing particles comprise teflon,
silica, alumina or mixtures thereof.
Description
TECHNICAL FIELD
[0001] The inventive composition creates a structured fluid which
improves the long-term stability, response time and visible
appearance of image displays which electrophoretically frustrate
total internal reflection (TIR).
BACKGROUND
[0002] In electrophoresis an ionically-charged particle moves
through a medium due to the influence of an applied electric field.
The concept of electrophoresis can be combined with the principles
of `Total Internal Reflectance` (TIR) to create addressable
displays. A suspension of particles can be used to controllably
frustrate TIR and switch the state of pixels in such displays in a
cotrolled manner. For example, an electromagnetic field can be
applied to move charged particles in the suspension through an
electrophoretic medium toward or away from an evanescent wave
region to frustrate TIR at selected pixel portions of the region.
In order for the electronic display to be useful the display should
have quick response times. Further it is desirable that there is
good contrast between the dispersed particles and the white
background and that the electrophoretically active suspension
remains stable.
[0003] It is known that repeated switching of a display which
utilizes electrophoretically-mobile particles can result in a
non-uniform distribution or movement of the particles, gradually
causing the formation of particle clusters which deteriorates the
quality of images produced by the display over time. An example is
found in Dalisa, A., "Electrophoretic Display Technology," IEEE
Transactions on Electron Devices, Vol. 24, 827-834, 1977; and Murau
et al, "The understanding and elimination of some suspension
instabilities in an electrophoretic display," J. Appl. Phys., Vol.
49, No. 9, September 1978, pp. 4820-4829. It has been shown that
such undesirable clustering can be reduced by encapsulating groups
of suspended particles in separate micro-fluidic regions. See for
example Nakamura et al, "Development of Electrophoretic Display
Using Microencapsulated Suspension," Society for Information
Display Symposium Proceedings, 1014-1017, 1998 and Drzaic et al, "A
Printed and Rollable Bistable Electronic Display," Society for
Information Display Symposium Proceedings, 1131-1134, 1998.
[0004] In summary it is desirable for an electronic display to have
long term stability, quick response time and high contrast between
the background and image being displayed. The invention has
discovered that certain compositions have a combination of physical
properties which overcome these obstacles in particular on
electrophoresis.
SUMMARY OF INVENTION
[0005] The inventive composition creates a structured fluid which
improves the response time, visible image and long-term image
stability of an electrophoretically-mobile particle display. The
composition comprises 1) a low refractive index liquid, which is
the electrophoretic medium 2) particles selected from the group
consisting of light absorbing particles such as pigments which are
charged, non-light absorbing uncharged particles which increase the
viscosity such as, teflon, silica, alumina and the like and
combinations thereof, 3) additives which include a) dispersants, b)
charging agents, c) surfactants (also interchangeable with the term
surface action agents), d) flocculating agents, e) polymers and f)
and combinations thereof.
[0006] The composition is used to improve response time to form a
display image after application of an electric field. The
composition forms a structured suspension of particles in which the
particles are stable from agglomeration. The particles interact
through colloidal forces controlled by the composition which
inhibits particle motion under low stress caused by gravity or by
the osmotic flow of ions in an electric field induced by reversing
the electric field as the display's pixels are switched between
reflective and non-reflective states, without encapsulating the ink
in isolated compartments.
BRIEF DESCRIPTION OF DRAWINGS
[0007] FIG. 1 is a fragmented, cross-sectional view, on a greatly
enlarged scale, of a portion of a prior art electrophoretically
frustrated TIR image display, depicting undesirable non-uniform
particle distribution.
[0008] FIG. 2A is a fragmented, cross-sectional view, on a greatly
enlarged scale, of a portion of a prior art electrophoretically
frustrated TIR image display, before application of an electric
field.
[0009] FIG. 2B depicts the FIG. 2A display after selective
application of an electric field.
[0010] FIG. 3B is a fragmented, cross-sectional view, on a greatly
enlarged scale, of a portion of one pixel of an electrophoretically
frustrated TIR image display in accordance with the invention,
before application of an electric field.
[0011] FIG. 3A depicts the FIG. 3B display after selective
application of an electric field.
DETAILED DESCRIPTION
[0012] FIG. 1 shows dilute mixtures in unrestricted
motion--historical prior art herein referred to as Case 1. FIG. 1
depicts a portion of a TIR image display which uses electrophoretic
dispersions to create an image. The upper polymeric sheet 42
contains an array of reflective microprisms 44. The sheet can be
constructed with prismatic geometry or my contain hemispherical
high refractive index transparent hemi-beads, as described in U.S.
patent application Ser. No. 10/086,349 filed 4 Mar. 2002 which is
incorporated herein by reference. A thin, continuous, transparent
electrode such as an indium tin oxide (ITO) coating 46 is applied
to the inward surfaces of prisms 44. A segmented electrode 50 is
applied to the inward surface of the bottom sheet, 48 to apply
separate voltages (corresponding to individual pixels) between each
adjacent pair of prisms 44. An electrophoresis medium--continuous
liquid 58, for example, a low refractive index, low viscosity,
electrically insulating liquid such as Fluorinert.TM.
perfluorinated hydrocarbon liquid available from 3M, St. Paul,
Minn. substantially fills the space between the sheets forming a
TIR interface between the two sheets 42 & 48. This mixture also
contains additives that interact with the particle surface to make
it become ionically charged. This primary composition is a
homogeneous dispersion of particles--suspension which will fill the
liquid 58 uniformly, and the concentration of particles 52 is
relatively low, in the order of 1% by weight. The particles 52 in
this composition Case 1 are well dispersed; they randomly move by
Brownian motion, and they will segregate from the liquid 58 under
the influence of gravity. The particle separation is many times (in
the order of ten times) greater than the particle size, so there
are very few particles near the surfaces of the reflective
micro-prisms 44.
[0013] A voltage source (not shown) is electrically connected
between the electrodes on the prism surface 46 and the bottom
segmented electrodes 48 to controllably apply a voltage across
selected pixel regions of liquid medium 58. Application of a
voltage across a selected pixel region electrophoretically moves
particles 52 (pigments) suspended within the selected region to
form a layer that begins within about 0.25 micron of the evanescent
wave zone adjacent the inward surfaces of the selected region's
prisms and extends about 5 microns into the region. When
electrophoretically moved as described, particles in the suspension
54, which have a higher refractive index than the surrounding
liquid 58 and are much smaller than a wavelength of light and
therefore substantially non-light-scattering, cause the layer to
have an effective refractive index that is substantially higher
than that of the surrounding liquid 58. This absorptive particle
layer causes absorption of the light as it passes through the upper
sheet. This gives the selected pixel region a dark appearance to an
observer who looks at outer surface of the microprism upper sheet
42. This process is slow (compared to Case 3-type compositions)
because a number of particles 52 must move a relatively long
distance to produce this optical effect. Application of an opposite
polarity voltage across the selected pixel region
electrophoretically moves the suspended particles 54 toward that
lower segmented electrode 50. As a result the particles 52 are out
of the evanescent wave zone and the light which passes through the
microprisms 44 undergoes TIR so the region has a white appearance
to an observer who looks at the sheet's outer surface.
[0014] Additional details of the construction of these displays and
optical characteristics of electrophoretically-frustrated TIR image
displays can be found in U.S. Pat. Nos. 6,064,784; 6,215,920;
6,304,365; 6,384,979; 6,437,921; and 6,452,734 all of which are
incorporated herein by reference; and, in the aforementioned U.S.
patent application Ser. No. 10/086,349.
[0015] The bottom electrode can be segmented to provide electrode
segments 50, as shown in FIG. 1. A controller (not shown) can then
be used to selectively apply a voltage to each pair of electrodes
in the segmented electrode array. Each electrode segment 50 (or
group of adjacent electrode segments) corresponds to an
individually controllable pixel.
[0016] Dispersed particles in the suspension 54 will tend to
agglomerate or stick together as they move near one another because
of van der Waals attractive forces. The dispersants are added to
the mixture to inhibit agglomeration, and they do this by forming a
barrier from electrostatic or osmotic pressure forces. However, the
dispersion 54 is inherently unstable. These lyophobic colloidal
dispersions require a great deal of mixing energy when being made.
They are thermodynamically unstable, but the dispersant barrier
helps to inhibit the ultimate breakdown that is agglomeration, size
growth and separation of the two phases. When these dispersions 54
are put in an electric field which moves the particles 52 they will
collide with tremendous force and this will tend to enhance
agglomeration. These dispersants as commonly used to stabilize
suspensions, that is provide a barrier to inhibit agglomeration
when particles collide with thermal energy, but they will not
provide a large enough barrier to prevent agglomeration with the
collision force induced by an electric field. Also, as the field is
reversed consecutively, electric field gradients will cause the
charged particles and ions to migrate between adjacent cells. As a
result, particles 52 in these Case 1 compositions will tend to
accumulate or cluster 56 in regions, and they will not readily
diffuse back to fill space uniformly. The particles in these Case 1
compositions will also tend to segregate from gravity driven motion
due to differences in density between the particles and liquid.
This will also tend to result in regions with high higher and lower
particle concentration. The motion of particles 52 in an electric
field gradient and the clustering 56 will also tend to enhance
agglomeration. The particles 52 in these Case 1 compositions will
rotate in the field gradient, and as they are packed into clusters
particles 56 will tend to arrange so that the part of the particle
with least repulsive forces are closest. The particle surfactant
coating may well be non-uniform and the charge distribution may be
non-uniform--hence, the particle motion and clustering will tend to
enhance contact between the parts of the particle that are most
likely to have strong attraction; so they will agglomerate. These
phenomena are illustrated in FIG. 1. Electrophoretically-frustrated
display can exhibit undesirable clustering of particles 56 in the
suspension 54 over time. More particularly, particles 52 tend to
form loose agglomerates, surrounded by regions of the
electrophoretic medium 58 containing relatively few suspended
particles 52. Such clustering often results in long-term
deterioration of the display's image quality and overall
performance.
[0017] FIGS. 2A and 2B show dilute mixtures in confined
compartments--prior art to minimize cluster formation herein
referred to as Case 2. FIGS. 2A and 2B depict a prior art technique
for reducing undesirable particle clustering with an ink
composition. This composition is similar to that in Case 1
composition in an electrophoretically-frustrated display having a
transparent upper `microprism` sheet 72 and a lower substrate sheet
78. The upper sheet 72 contains an array of parallel reflective
microstructred prisms 74. The tip of the microprisms 74 are
connected to the lower sheet 78 as illustrated. This forms an
encapsulated channel 88 between opposed facets of each adjacent
pair of prisms. The encapsulated channels 88 will prevent particle
migration between adjacent cells, and it can also inhibit particle
sedimentation, and this will reduce formation of particle clusters.
Each channel is filled with an electrophoresis liquid medium 80,
forming a TIR interface between the upper microprism sheet 72 and
the continuous liquid medium 80. This continuous liquid medium 80
contains a finely dispersed suspension 86 of pigment particles 84.
A thin transparent electrode such as ITO 76 is applied to the
inward surface of the upper microprism sheet 72. A segmented
electrode 82 is applied to the inward surface of the lower sheet
78, to create separate pixel regions corresponding to each channel
(or a group of adjacent channels 88).
[0018] A voltage source (not shown) is electrically connected
between the electrodes on the prism surface upper ITO coated
electrode 76 and the bottom sheet segmented electrodes 82 to
controllably apply a voltage across selected pixel regions of
liquid medium 80. Application of a voltage across a selected pixel
region electrophoretically moves pigment particles 84 suspended
within the selected region to form a layer that begins within about
0.25 micron of the evanescent wave zone adjacent the inward
surfaces of the selected region's prisms and extends about 5
microns into the region. When electrophoretically moved as
described, particles in the suspension 86, which have a higher
refractive index than the surrounding fluid 80 and are much smaller
than a wavelength of light and therefore substantially
non-light-scattering, cause the layer to have an effective
refractive index that is substantially higher than that of the
surrounding liquid 80. This absorptive particle layer 90 causes
absorption of the light ray 70 as it passes through the upper sheet
72. This gives the selected pixel region a dark appearance to an
observer who looks at outer surface of the upper `microprism` sheet
70. This process is slow (compared to Case 3--type compositions)
because a number of pigment particles 84 must move a relatively
long distance to produce this optical effect. Application of an
opposite polarity voltage across the selected pixel region
electrophoretically moves the suspension of pigment particles 86
toward that lower segmented electrode 82. As a result the pigment
particles 84 are out of the evanescent wave zone and the light
which passes through the microprisms 74 undergoes TIR 68 so the
region has a white appearance to an observer who looks at the
sheets outer surface.
[0019] In Case 2, although encapsulation of the ink into
compartments keeps the suspension of particles 86 within separate
channels 88 and reduces undesirable clustering, it may in some
cases be impractical to fabricate, fill or maintain channels 88. In
Case 2 the encapsulation may not completely eliminate particle
clustering and agglomeration in the ink because they can segregate
within a cell, and the strong electric field will still increase
the force of particle collisions.
[0020] The inventive composition herein, Case 3, provides a
suspension 110 with structure to minimize cluster formation, and
improve contrast and speed. The composition of the invention
creates a stable dispersion with a colloidal structure where the
light absorbing particles 100 are charged. The composition
comprises 1) a low refractive index liquid 104 which is the
electrophoretic medium; 2) particles 100 including light absorbing
particles such as pigments which are charged and low light
absorbing uncharged particles which increase the viscosity and
provide part of the interactive or structured network, such as,
teflon, silica, alumina and the like; and 3) additives which
include a) dispersants, b) charging agents, c) surfactants, d)
flocculating agents, e) polymers and f) and combination
thereof.
[0021] The composition also provides for good contrast of a dark
image in a white background, and a rapid response time to form the
image after application of the electric field. The composition may
be a mixture of particles which form a very dark color, preferably
black. It is preferable to have a dark image against a light such
as white background. The concentration and nature of components in
the composition are adjusted to form a structured fluid where the
particles interact with each other and the other components so they
will not readily flow under a low stress, but will move rapidly in
an electric field to form an image.
[0022] The composition contains a low refractive index liquid which
include fluorinated liquids, Fluorinert perfluorinated hydrocarbon
liquid manufactured by 3M, St. Paul, Minn., Krytox Oil, a
perfluoropolyether manufactured by DuPont performance Lubricants,
Wilmington, Del. and the like. The low refractive index liquid may
be used in combinations thereof. The low refractive index has a low
dielectric constant in the range of about 1 to about 20, preferably
about 1 to about 10 and more preferably about 1 to about 5. The low
dielectric constant reduces the overall conductivity of the
composition. The low reactive index liquid includes polar,
non-polar and mixtures thereof, preferably the low reactive index
liquid is non-polar. The liquid is in the composition in the range
of about 10 wt. % to about 95 wt. %, preferably about 30 wt. % to
about 60 wt. % of the composition. The low refractive index liquid
has a molecular weight in the range of about 100 to about 5,000,
preferably about 200 to about 5000 and more preferably about 500 to
about 1000 and a viscosity in the range of about 1 centipoise to
about 100 centipoise preferably about 1 centipoise to about 10
centipoise. The refractive index difference between the polymer
transparent front sheet of the display and liquid is as large as
possible, at least about 0.15 and preferably at least 0.30. The
volatility of the liquid should be as low as possible while
maintaining a low viscosity. The chemical structures of exemplary
liquids are shown below. Paritally fluorinated fluids can also be
used.
Structure of Krytox Oil is as follows:
[0023] Krytox Oil TFL 8896, Polyhexafluoropropylene oxide ##STR1##
1,1,1,2,2,3,3-Heptafluoro-3-pentafluoroethyloxy-propane for n=1 The
Structure of Fluorinert Oil is as follows:
[0024] Fluorinert FC-75, perfluorinated fluid ##STR2##
1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-Octadecafluoro-octane
[0025] It is preferable that the liquid has as a low refractive
index as possible (per fluorinated liquids have the lowest
refractive index). The composition contains additives which are
soluble in the liquid and which stabilize the suspended particles
(prevent agglomeration) and cause the particles to become charged
so they are electrophoretically active.
[0026] The composition contains particles which include light
absorbing particles, very low light absorbing particles and/or
non-light absorbing particles which increase the viscosity of the
overall composition and provide part of structural network, and
mixtures thereof. In one embodiment of the invention the particles
in the composition are preferably light absorbing particles. In
another embodiment of the invention the particles in the
composition are preferably light absorbing particles and non-light
absorbing particles. The light absorbing particles include
pigments, metals, mixtures thereof and the like; they are finely
dispersed in the liquid electrophoretic medium, and they are
charged. The non-light absorbing particles are finely dispersed in
the liquid, and they include organic polymers such as teflon,
polystyrene, nylon, polycarbonate and the like and inorganic
compounds such as silica, alumina, calcium carbonate, clays
(kaolin, bentonite, montmorillonite, etc.) and the like and
combinations thereof, and these particles are added to help create
a structured colloidal dispersion. However, they are not charged in
the composition because they do not contribute to the optical
effects. These particles may be spheroidal, polyhedral or have a
high aspect ratio like needles or rods which can enhance the
structure of the mixture. The uncharged particles are in the
composition in the range of about 0 wt. % to about 60 wt. % and
preferably about 2 wt. % to about 20 wt. % of the composition.
[0027] Pigments are colored particles which may be organic or
inorganic in nature. Pigments can be broadly classified into
colored and white pigments. White pigments are inorganic while
colored pigments can be organic or inorganic. The pigments include
quinacridones which are red or copper colored, pthalocyanines which
are blue, or carbon black, iron oxide or aniline black which are
black and combinations thereof. Strongly colored pigments are
preferable since they offer high contrast with the background. A
composition containing equal weight mixtures of a quinacridone and
pthalocyanine pigment produce a very dark color which is especially
suitable for many displays. The pigment particles can be a mixture
of more than one and are in the ratio in the range of 50 wt. % to
about 50 wt. %, in one embodiment about 30 wt. % to about 70 wt. %,
and in another embodiment about 20 wt. % to about 80 wt. %. For the
purpose of using pigments in outdoor display applications it is
desirable that they possess general characteristics as follows.
TABLE-US-00001 Property Desired Color Dark Light Fastness High Ease
of Dispesion High Presence of metal ion Acceptable Particle size
<300 microns, and preferably <450 microns.
[0028] The color and light fastness depends on the structure of the
pigment, and the pigments with the above properties are desired.
The surface chemistry of the pigment controls its dispersion, and
particle size.
[0029] The particle may contain amine functionality, nitrogen
containing molecules that impart bascity, acidic functional groups
and the like. The particle may contain combinations of
functionality. Exemplary amine functionality are shown in the
structures below, the quinacridone and pthalocyanine pigments.
[0030] The quinacridone pigment, NRT-796D-Monastral Red-B is
represented by the structure as follows ##STR3##
[0031] The pthalocyanine pigment, Cromophtal Blue A3R is
represented by the structure as follows: ##STR4##
[0032] Examples of pigment particles with exemplary nitrogen
containing segments in the structure imparting a degree of basicity
to the pigment surface include aniline black and the like. Carbon
black has acidic groups on its surface, including carboxylic acid
and phenolic groups, and other commercial pigments have acidic
functional groups. The surface chemical functional groups are the
sites for interaction with the other components in the composition.
Pigments are further modified by surface treatments. These surface
treatments in turn impart additional functional groups for
interactions with other components in the composition.
[0033] The particle concentration in the composition is adjusted to
obtain a particle separation which promotes particle/particle
interactions (long range), and generally this results in an ordered
arrangement of particles, that is a colloidal structured fluid. The
particles are in the composition in the range of about 1 wt. % to
about 75 wt. %, preferably about 10 wt. % to about 60 wt. % of the
composition. The interactions include coulombic interactions,
steric interactions, osmotic pressure interactions and the like
induced from absorbed or attached surfactants, depletion force
interactions from polymers dissolved in the liquid, and attractive
forces between particles in a weakly flocculated state. These
interactions are facilitated by other components in the composition
and these components also assist in preventing particle
agglomeration. The particle spacing (needed for desired
particle-particle interactions) depends upon the concentration of
other components which interact collectively to produce the
interactive forces. The desired spacing depends upon the balance of
forces which restrict motion and inhibit segregation of particles
with the ability to move quickly in response to an electric field
resulting in fast response time.
[0034] The composition may contain two or more sets of particles
with different particle size distributions to improve the structure
by enabling a more efficient packing arrangement. This enables a
higher loading of particles and smaller separation distances
between particles, and then stronger interactions between
particles. The viscosity is higher, and the structuring of the
fluid is enhanced. This helps reduce particle migration, decreases
segregation of particles and decreases the tendency of particles to
form clusters. The higher particle loading also improves the dark
color density when the dispersion is in the evanescent wave zone
near the surface of the display, and this improves the image
quality.
[0035] The composition includes dispersants which are soluble in
the liquid. The dispersants include Krytox.TM. 157-FSL, Krytox.TM.
157-FSM or Krytox.TM. 157-FSH fluorinated oil (respectively having
specified molecular weights of approximately 2500, 3500-4000 and
7000-7500, CAS Registry No. 860164-51-4, DuPont Performance
Lubricants, Wilmington, Del. 19880-0023); they are shown below, and
Zonyl fluorosurfactants, or Forafac fluorinated surfactants, DuPont
Chemical Company, 1007 Market street, Wilmington, Del. 19898.
Combinations of dispersants may be used.
[0036] The dispersant concentration in the composition depends upon
the concentration of pigment particles, and on the other components
in the mixture. The weight ratio of dispersant to pigment in the
composition is in the range of about 0.1 to about 3.0, and
preferably about 1.0 to about 2.0. The dispersant in the
composition is in the range of about 0.001 wt. % to about 70 wt. %,
preferably about 2 wt. % to about 40 wt. % of the composition.
[0037] The Krytox 157--FSH, Perfluroalkylpolyethercarboxylicacid
(Mw 5000-7000) is represented as follows: ##STR5##
2,3,3,3-Tetrafluoro-2-heptafluoropropyloxy-propionic acid where n=1
Krytox.TM. 157-FSL (lower average molecular weight of above FSH, Mw
3500)
[0038] The dispersant interacts with the surface of the particle to
form a strong bond which anchors it to the surface, and the tail of
the surfactant is highly soluble so it creates a barrier (generally
through osmotic pressure) to prevent agglomeration with other
particles. More than one dispersant can be used. The dispersant
depends on the surface chemistry of the particle; the selection is
made to optimize the interactions. For example, a dispersant with
an acid functional group, like carboxylic acid, might be chosen for
a particle containing basic functionality, like the quinacridone or
pthalocyanine pigments, to obtain strong interactions. Likewise, a
dispersant with a basic amine functional group, might be chosen for
carbon black. At least one of the dispersants interacts by
acid-base interactions to produce an ion pair (salt) with the
surface of the light absorbing particle.
[0039] In one embodiment, the particle surfaces are almost
completely to completely covered with dispersant. Some of the
dispersant needs to dissociate from the surface so the light
absorbing particle is charged. Preferably the surface of the
particle and dispersant should form pairs which saturate the
surface, but upon saturation also enable a small degree of
dissociation. This can be facilitated by allowing close packing
with some steric constraint upon saturation. The charged particles
will have electrostatic interactions as they move near each other.
This will provide a barrier which inhibits agglomeration, and in
conjunction with steric (osmotic pressure) interactions it will
make a very stable suspension where the particles will not
agglomerate.
[0040] The degree of charging can be adjusted by varying the
dispersant in the composition of the dispersion. The degree of
dissociation will depend on the combination of solubility in the
liquid, the strength of acid-base interaction, the molecular shape
of dispersant and the molecular structure of the particle surface.
Charging is also improved when the dissociated dispersants form
micelles which help minimize the recombination of the ions, and
this results is a higher degree of overall dissociation. In liquids
with a low dielectric constant the double layer around charged
particles (distance from the particle surface to charge neutrality)
tends to be very large. This is because there are a very small
number of ions in the liquid. As the particle spacing is decreased
the charged particles will interact with each other, and this will
create a suspension with a highly ordered structure. The spacing
needed for the interaction will depend upon the magnitude of
particle charge and the number of excess ions in solution, and
these are controlled by the concentration and characteristics of
the components used. Higher charging and lower concentrations of
excess ions increase the strength of the interactions. The
particles in this suspension interact with all nearest neighbors
and this interaction inhibits their movement under low shear.
[0041] In one embodiment, dispersants with either acidic or basic
functional groups chosen to interact with the complementary basic
or acidic functional groups on the surface of the light absorbing
particle are used. This results in a composition containing charged
light absorbing particles and only the counter ions for those
charged groups, hence no excess ions. As a result the conductivity
of the mixture is very low; this improves the structure and
performance of the composition. It is preferable that the
conductivity of the composition is low so the electrical power
requirements of the device is minimized.
[0042] In another embodiment, the dispersant contains functional
groups on opposite ends of the backbone of the molecule. This
enables it to become tethered to two different particles, and this
prevents particle migration. The functional groups are chosen so
they have strong interactions with the functional groups on the
surfaces of the particles. The length of the molecule is chosen to
allow some flexibility in movement, and in particular to allow the
particle spacing to become compressed when the dispersion is placed
in an electric field. The molecule also is long enough, and
contains bulky enough and highly soluble branches along its
backbone, ranging from methyl to dendritic structures which entrain
the liquid and prevent particle agglomeration by osmotic pressure.
In fluorinated liquids a fluorinated backbone is more soluble, so
it is preferred in this embodiment. The concentration of this
bi-functional dispersant is kept low enough so some functional
groups on the surface of the light absorbing particles are
available for interactions with mono-functional dispersants. These
mono-functional dispersants contain acidic or basic functional
groups which form salt pairs with the complementary acidic or basic
functional groups on the light absorbing particle surfaces, and
when they dissociate, the particle becomes charged. This
composition of bi-functional and mono-functional dispersants
combined with particles and liquid form an interlocked network. The
particles will not form agglomerates or clusters nor will they
migrate, and the network can expand or contract when placed in an
electric field.
[0043] In another embodiment, the particles in the composition are
loosely flocculated, and they form a network. The ordered
arrangement of particles depends on the size and packing of the
particles. Flocculation occurs when the particle separation is less
than the distance of van der Waals attractive forces which depends
on the particle size and physical characteristics of the particle.
These particles are still dispersed well enough to prevent tight
agglomeration. This is accomplished by using a low molecular weight
dispersant which covers the particle surface thoroughly. This
dispersant is soluble in the liquid and has a short tail, in the
range of about 4 to about 20 carbon atoms in length, and contains
functional groups similar to those described previously, such as
amines or carboxylic acid, and the like, which bond strongly onto
the functional groups on the particle surfaces. Fluorinated
dispersant molecules would be more soluble, and are preferred for
fluorinated liquids. The composition may contain combinations of
such dispersants. The flocculating agents are in the composition in
the range of about 0% to the range of about 0.001 wt. % to about 70
wt. %, preferably about 2 wt. % to about 40 wt. % of the
composition.
[0044] The dispersants are chosen so all particles are well
dispersed, but only the light absorbing particles are charged.
[0045] If the particles are well dispersed, and the available space
(in the liquid) is occupied by dissolved non-adsorbing polymer or
other uncharged dispersed particles as previously described this
can also create a suspension with a highly ordered structure. A
polymer which is highly soluble in the liquid can cause the
particles to form a highly ordered structured from attractive
depletion forces and the effectiveness will depend on the relative
size of the polymer radius of gyration and particle size combined
with the concentration of each. This type of interaction can result
is ordering with particle volume fractions as low as a few volume
percent. At high concentrations non-adsorbing polymers in solution
can stabilize particle suspensions (prevent agglomeration), and
this can also result in an ordered structure at higher particle
concentrations.
[0046] The polymers include highly soluble forms of polyethylene,
polypropylene, polyisobutylene, polystyrene or the like which do
not adsorb onto the particles in the composition (no functional
groups to interact with the functional groups on the particle
surfaces), and they have a high molecular weight such that the
radius of gyration is close to the radius of the light absorbing
particles in the composition; these may be co-polymers or
homo-polymers with branching to increase the entrainment of
solvent--for fluorinated liquids partly or completely fluorinated
polymers would be more soluble--in this embodiment it is preferred.
The polymers would be in the molecular weight range from about one
thousand Dalton (Da).to about one million Da., preferably about ten
thousand Da. to about a few hundred thousand Da. The polymers may
be used in combination. The polymer is in the composition in the
range of about 0.1 wt. % to about 70 wt. %, preferably about 1 wt.
% to about 20 wt. % of the composition.
[0047] The composition may also include rheology control agents.
These are soluble polymer molecules which become swollen by the
liquid, and this causes an increase in the viscosity of the liquid;
this decreases the mobility of particles. This helps prevent
particle segregation and enhances the structure of the fluid. The
swelling of the polymer will vary inversely with temperature, and
this counterbalances the change in viscosity of the liquid with
temperature. This helps maintain constant fluid flow properties
with temperature changes, and this results in more consistent
response time with variation of temperature. The rheology control
agents include ethylene plus propylene copolymers, styrene plus
butadiene copolymers, polymethacrylates, polyisobutenes and the
like. Combinations may be used. The rheology agents are soluble in
the liquid; fluorinated polymers would be more soluble in
fluorinated liquids so it would be preferred in that embodiment.
The polymers have molecular weights in the range from about 10,000
Da to about one million Da. The rheology control agents are in the
composition in the range of about 0% (not present) to the range of
about 0.01 wt. % to about 25 wt. %, preferably about 0.5 wt. % to
about 15 wt. % of the composition.
[0048] The composition may also include surface active agents
(surfactants) which have a different function than the dispersants.
These surfactants act as charging agents by forming salt pairs with
larger molecules, like dispersants, in the composition, or they
facilitate charging by forming micelles possibly in conjunction
with other components in the composition. They increase particle
charging by improving the dissociation of salt pairs by mediating
the charge on the ion or associating with the ion to decrease the
tendency to recombine with the counter-ion. They may also act as
co-dispersants and occupy sites on the particle surface to help
improve the total surface coverage. The surfactants include soluble
small molecules with short tails and some polar groups, such as
hydroxyl, substituted aromatics, carboxylic, amine, amide, as well
as salts and aromatic groups and the like. The surfactants may be
used in combination. The surfactants are in the composition in the
range of about 0 wt. % to about 25 wt. % and in another embodiment
in the range of about 0.01 wt. % to about 20 wt. % of the
composition.
[0049] In another embodiment, the composition is used to create a
suspension which remains unagglomerated (no cluster formation)
under conditions of operation without forming a structured fluid.
In this case, the colloidal suspension is stabilized by having a
tightly packed dispersant (surfactant) layer which is very strongly
bound to the particle surface. This dispersant also has a high
molecular weight tail that is very soluble in the liquid, and this
inhibits agglomeration under severe conditions without forming a
structured fluid. These suspensions will become structured at
higher particle loadings, but the response time is too slow or
optical properties are inadequate because particle motion is too
severely limited. For example, when very high concentrations of
dispersant (surfactant) are used, a highly structured dispersant
(surfactant) layer can form on the surface of the solid. This
tightly packed layer can include several dispersants, chosen to
maximize coverage and strength of bonding to the surface of the
particle. The light absorbing particles are charged in this
composition because some of the dispersant which formed salt pairs
on the particle surfaces has become dissociated, leaving a net
charge.
[0050] Overall the total amount of all the additives in the
composition are in the range of about 0.1 wt. % to about 60 wt. %,
preferably about 1 wt. % to about 40 wt. % and more preferably
about 5 wt. % o about 30 wt. % of the composition.
[0051] The colloidal dispersion of the composition has a structure
which inhibits the particle migration from the low stress
associated with gravitational segregation or caused by field
gradients associated with reversing the electric field. This
structure also inhibits aggregation of particles caused by strong
collisions driven by a high electric field. The structured array of
particles will become compressed and pushed away from one electrode
and toward the other one when the field is applied. However, the
forces associated with the structure will inhibit the compression
of particles, and the higher particle concentration results in
shorter distance traveled, therefore lower velocities will be
reached, and there will be a reduction in agglomeration caused by
the electric field.
[0052] The structure will inhibit particle motion, but this is
balanced by the optical properties required of the device. The
composition contains particles that are spaced more closely
together, and this increases particles interactions. It will also
result in particles being near the prismatic surface. This is
illustrated by FIG. 3B which shows a structured suspension 110
which has a uniform dispersion of particles 100 closely spaced. For
example, for particles 100 with a diameter of about 150 nm the
spacing will range from about 300 nm at a volume fraction of about
0.1 to about 130 nm for a volume fraction of about 0.25 and about
40 nm for a volume fraction of about 0.5. As the volume fraction
increases the spacing becomes smaller than the particle diameter
and the particles 100 will be well within about 0.25 micron of the
evanescent wave zone adjacent the inward surfaces, of the prisms
94. As a result the particles 100 will absorb the incident light
108 causing a refractive index mismatch which frustrates TIR,
giving the depicted pixel region a dark appearance to an observer
who looks at sheet's 92 outer surface.
[0053] When the electric field is applied, the structured
suspension 110 of the composition becomes compressed 102 and moves
away from the electrode surface 96; this is illustrated in FIG. 3A.
For example, when a voltage source, not shown, is electrically
connected between the upper and lower electrodes 96 and 98 to
controllably apply a voltage across the uniform suspension 110, the
spacing between particles 100 becomes reduced as the particles, in
the uniform suspension 110, are squeezed away from the electrode 96
surface. This leaves a liquid film of low refractive index fluid
104 between the inward surface of upper sheet 92 and the compressed
suspension 102, and it is sufficiently thick approximately 0.25
microns that it enables substantially all of the evanescent wave to
be confined to a particle-free region of fluid and thus cause TIR
106, such that light which passes through the upper sheet 92 is
reflected by TIR 106 at the interface, giving a white appearance to
an observer who looks at sheet's 92 outward surface. It only
requires a small displacement of the particles 100 in the
composition to create a 0.25 micron thick film of liquid 104. When
as depicted in FIG. 3b, the field is reversed, the particles 100
are pulled into the evanescent region and they frustrate the TIR
108 causing the depicted pixel region a dark appearance to an
observer who looks at the outer surface of the sheet 92.
[0054] The movement of particles into and out of the evanescent
wave zone must be very rapid so that the transition of pixel color
from white to dark happens very fast. The structure of the
dispersion composition must be adequately strong to prevent
migration under low stress; it must inhibit strong collisions under
a high electric field, but it cannot reduce the speed of particle
motion into and out of the evanescent zone near the electrode.
[0055] The particle charge which helps create the ordered structure
of the dispersion also causes electrophoretic motion of the
particles. As the particle charge increases the electrophoretic
velocity into and out of the evanescent zone will increase. These
two effects--columbic interactions which promote a rigid structure
and electrophoretic particle motion are linked together. The
composition which promotes an ordered structure, including charging
and close particle spacing also promotes the fast response to the
electric field. The close particle spacing enables the change in
color with minimal movement of particles, and the particle charging
enables electrophoresis which becomes faster as the charge
increases.
Specific Embodiment
[0056] The following examples demonstrate the composition and
advantages of the present invention.
[0057] The composition will control the structure, and the
structure can be measured using rheology. Rheology is the
measurement of the flow properties of a fluid. When a stress is
applied to a fluid it will flow, and the measurement of shear
stress with rate of shear will show the characteristics of that
fluid. In particular, the rheology of a dispersion will show how
the particles move in the liquid. The stress can be applied as an
oscillation or as a continuous stress, and these measurements show
different aspects about the structure.
[0058] The composition of the mixture can be adjusted to modify its
colloidal structure and obtain a suspension which does not form
agglomerates or clusters, but still has a rapid response to the
application of a low electric field; this dispersion has a specific
type of rheology which helps to characterize its structure. This is
illustrated in the following examples.
[0059] Three mixtures R, S and V containing pigments, liquid and
surfactant were made as described below.
Formulation 1
Mixture R:
[0060] 25.0w % Pigment [0061] 12.5% w NRT-796D-Monastral Red-B
[0062] 12.5% w Cromophtal Blue A3R [0063] 25.0% w Krytox.TM.
157-FSH [0064] 50.0% w Krytox.TM. Oil Mixture S: [0065] 43.0 w %
Pigment [0066] 21.5% w NRT-796D-Monastral Red-B [0067] 21.5% w
Cromophtal Blue A3R [0068] 14.0% w Krytox.TM. 157-FSH [0069] 43.0%
w Krytox.TM. Oil Mixture V: [0070] 34.0% w Pigment [0071] 17.0% w
NRT-796D-Monastral Red-B [0072] 17.0% w Cromophtal Blue A3R [0073]
19.5% w Krytox.TM. 157-FSH [0074] 46.5% w Krytox.TM. Oil
[0075] Mixture V was made by mixing equal portions of R and S.
These mixtures were each evaluated using a device with a design
illustrated in FIG. 3A, and constructed with 25 .mu.m prisms
separated from a conductive substrate such that the average gap
thickness was 75 .mu.m. The mixtures were introduced into the gap
between the microstructured surface and the rear substrate and the
reflectance of light from the surface was measured when the device
was subjected to a 1 Hz, 50 volt electrical pulse. The results of
this measurement for the 3 mixtures are shown in FIG. 4.
[0076] The graph in FIG. 4 shows that mixture S which has the
highest pigment loading has the slowest response to the field.
Mixture R which has the lowest pigment loading is faster than S,
but mixture V has the fastest response. This is unexpected; one
might expect that the response or speed of mixture V would be
between S and R--since it is a mixture of the two. Further, on
inspection of the components in the mixtures, mixture V has a
higher concentration of components that would tend to increase the
viscosity of the fluid. Hence one might expect that it should have
a slower response, but it is faster.
[0077] This result was investigated further from rheology
measurements. A Carrimed CSL controlled stress rheometer was used
for these measurements. A cone and plate configuration was used,
and the measurements were done at 25.degree. C. The results of
these measurements are shown in the plot in FIG. 5:
[0078] The upper curve in FIG. 5--rko179 is sample S; the middle
curve, 187180-2 is sample V, and the bottom curve, rko-178 is
sample R. The viscosity varies with shear rate in all cases, so
these mixtures are non-Newtonian, and this indicates that the
particle systems have some structure, or that the particles are
interacting with each other. The higher viscosity and greater
change with shear rate indicates that there is a greater degree of
structure. Another way of interpreting this data is to fit it to a
model which has parameters that relate to physical characteristics
of the suspension. This data was fitted to two conventional models,
the Herschel-Buckley model and the Cross model; the fitted
parameters are shown in the Table I below: TABLE-US-00002 TABLE I
Cross model H_B model zero shear Infinite Yield sample Pa-s shear,
Pa-s stress, Pa Rate index V 27.73 3.394 3.131 .8653 R 1.874 .6769
1.291 .9683 S .sup. 4.81E05 24.43 24.71 .7026
[0079] The zero shear and infinite shear parameters represent the
initial, unperturbed viscosity and viscosity of the continuous
liquid phase respectively; the yield stress represents how much
force is required to initiate movement--break a structure, and the
rate index indicates how close to Newtonian (totally unstructured)
the mixture is. Hence the sample with the highest concentration of
pigment, sample S, has the strongest structure, and the sample with
lowest pigment loading, sample R, has the weakest structure.
[0080] This suggests that the intermediate level of structure is
needed for the fastest response or movement of pigments in an
electric field. A structure which is too strong impedes motion and
this is what one might expect. However, the mixture with a weaker
structure responds more slowly, and this is contrary to
expectations. Perhaps the particles in the intermediate structured
fluid can move in tandem and hence faster than those in the less
structured fluid which may not be able to move in tandem because
the structure is too weak. It's also possible that this is due to a
combination of factors such as particle charge, inter-particle
interactions and ratio of forces--which squeeze the liquid to push
it out when the structured colloid is compressed.
[0081] The structure must also inhibit particle motion under
weaker, but long term stresses, such as gravity and diffusion
induced by field gradients. Hence the mixture must be adjusted to
reach the right combination, including particle spacing, particle
charging, colloidal stability and liquid viscosity--in combination
with the ability to move quickly in a strong field while remaining
immobile in low shear.
[0082] As will be apparent to those skilled in the art in light of
the foregoing disclosure, many alterations and modifications are
possible in the practice of this invention without departing from
the spirit or scope thereof. The scope of the invention is to be
construed in accordance with the substance defined by the following
claims.
* * * * *